An in-mold foam molded product obtained by packing pre-expanded polypropylene resin particles in a mold and hot-forming the particles with steam or other means has features such as an arbitrary shape, lightweight, and heat-insulating properties that are advantages of the in-mold foam molded product.
The in-mold foam molded product obtained by using the pre-expanded polypropylene resin particles is excellent in chemical resistance, heat resistance, and strain recovery after compression as compared with an in-mold foam molded product obtained by using expanded polystyrene resin particles, and is also excellent in dimensional precision, heat resistance, and compressive strength as compared with an in-mold foam molded product obtained by using expanded polyethylene resin particles.
On account of these features, the in-mold foam molded product obtained by using the expanded polypropylene resin particles has been used for various purposes, for example, a heat insulating material, a shock absorbing packing material, an automobile interior member, and a core material for an automobile bumper.
A conventional method for manufacturing the pre-expanded polypropylene resin particles is exemplified by a method of pressing an inorganic gas such as air into pre-expanded polypropylene resin particles in advance (what is called an “internal pressure application method”) and performing in-mold foam molding and a method of packing pre-expanded polypropylene resin particles in a mold while compressing the particles and performing in-mold foam molding (what is called a “compression packing method”).
However, the internal pressure application method has problems of requiring a large pressure device and requiring time for applying internal pressure. The compression packing method also has problems of requiring an apparatus for pressurizing and packing pre-expanded particles and requiring modification of an existing molding machine.
Because of these problems, a method of using pre-expanded polypropylene resin particles without pretreatment and performing in-mold foam molding may be employed.
However, the method of using pre-expanded polypropylene resin particles without pretreatment and performing in-mold foam molding is difficult to produce a molded product having a beautiful surface. For example, wrinkles remain after drying and shrinkage fails to return after drying because the molded product is likely to be deformed after molding.
In order to solve these problems, various methods have been studied using pre-expanded polypropylene resin particles without pretreatment and performing in-mold foam molding.
Patent Document 1 discloses a method of disposing an in-mold foam molded product of polyolefin resin in a temperature environment 25 to 50° C. lower than a melting point of the substrate resin when the volume of the molded product after released from a mold is 70 to 110% of the volume in the mold and leaving the molded product until the temperature of the molded product reaches at least the environmental temperature.
Patent Document 2 discloses a method of aging an in-mold foam molded product of polyolefin resin after released from a mold by using an aging apparatus with a carriage at an environmental temperature of 90 to 120° C.
Drying the in-mold foam molded product of polypropylene resin typically employs a drying chamber at 60 to 80° C., and thus the methods of Patent Documents 1 and 2 unfortunately require a special drying chamber or apparatus.
Other studies focus the melting energy of pre-expanded polypropylene resin particles at a high temperature peak of a DSC curve, as below.
Patent Document 3 discloses a method of using pre-expanded polypropylene resin particles that have a melting energy of 8 to 12 J/g at a high temperature peak of a DSC curve in a condition of a bulk density of 0.04 g/cc or more and have a melting energy of 8 J/g or more at a high temperature peak in a condition of a bulk density of less than 0.04 g/cc.
Patent Document 4 discloses a method of using pre-expanded polypropylene resin particles that have a melting energy of 8 to 20 J/g at a high temperature peak in a condition of a weight of 3 mg or more and a bulk density of 0.04 g/cc or more and have a melting energy of 8 J/g or more at a high temperature peak in a condition of a bulk density of less than 0.04 g/cc.
Patent Document 5 discloses a method of using pre-expanded polypropylene resin particles that have a melting energy of 8 to 20 J/g at a high temperature peak in a condition of a cell size of 3 mm or less and a bulk density of 0.04 g/cc or more and have a melting energy of 8 J/g or more at a high temperature peak in a condition of a bulk density of less than 0.04 g/cc.